Description of the Prior Art
The conventional structure of cooking surfaces is well known, comprising, in the most recent embodiments, separately controlled sources having heating elements constituted either by electrical resistors or, preferably, by what are called halogen lamps, mounted in a support whose base is formed by an adequate thickness of insulating material having reflective properties for the infrared radiation emitted by the heating elements, and the upper part whereof is formed by a sheet of glass on which rest the receptacles containing the foodstuffs to be cooked, directly over the various sources.
The upper sheet may be produced from laminated glass, but in this case cannot generally withstand the high temperature radiated by the heating elements. It is for this reason that, in modern cooking surfaces, the said sheet is constituted by a sheet of vitroceramic material, which is substantially stronger, without consequently impeding the transmission of the radiation originating from the heating elements.
Furthermore, various embodiments of control keys are known, for example from Patent Publications DE-A-2 805 527 and EP-A-0 042 167 which, by a contact action of the user's finger, provide the connection to electrical current of the heating elements, particularly by successive stages or in a regularly increasing or decreasing manner, as the case may be, regulating the efficacy and intensity of the desired heating. These keys may in particular be embodied in the form of electromechanically operating push-down members which require the employment of sealing means at their passage through the sheet of glass, for example with the aid of a flexible film of self-adhesive plastic or the like. However, this solution causes problems of reliability, and especially of service life, the movements of the keys and the flexions of the sealing film resulting therefrom making it impossible to ensure a satisfactory durability over time. Moreover, the presence on the sheet of receptacles which may be very hot is liable to result in the rapid deterioration of the plastic film and to an insulation failure, and hence short circuits in the electrical assembly actuated by the keys.
Another solution consists in using sensitive keys employing resistor or capacitor technology, of the type known for their use in, for example, channel selection in a television receiver or in an audiophonic system. In this case, each key is formed by a thin layer of metallisation deposited on the surface of the glass sheet which possesses, underneath, an electronic circuit comprising one of the armatures of two separate capacitors, the other armatures, mounted in series, being formed by the exterior metallisation layer. The two capacitors, for which the glass sheet fitted between their armatures plays the part of a dielectric material, are disposed in a circuit mounted under the sheet and sending an appropriate signal to an amplifier. When the finger is placed in contact with the outer metallisation layer on the top of the sheet of the key in question, the circuit is earthed via the body of the user himself, between the two capacitors in series, the amplifier then detecting a variation in the supplied signal which makes it possible to control, in the desired direction, the power supply to the heating source associated with the key.
However, though such a system is simple in design and of great reliability, it is unsuitable in the case of cooking surfaces, where the upper working surface is formed by a sheet of vitroceramic glass. This is because, in this case, it is impossible to carry out local metallisation of the upper face of the glass sheet, the latter not permitting a suitable bond with the deposited layer. It is thus essential, if it is desired to employ the conventional solution, to pierce the sheet in order to be able to touch the armature of the capacitor, which is then bonded or otherwise fixed beneath the lower face of this sheet, which causes problems of sealing and of aesthetic appearance. Moreover, in the event of splashing of liquid during cooking or of soiling directly over the key, or accidental contacts with the latter, the heating source will be lit or extinguished at the wrong time, which may have damaging consequences.
In order to remedy the above disadvantages, a method has been disclosed whereby the upper sheet of the apparatus is produced with the aid of a mixed structure comprising, above the heating sources, an appropriate thickness of vitroceramic glass and, attached to the side of the latter, a narrow lateral strip bearing the sensitive control keys, this strip of laminated glass then being able to undergo metallisation of the keys, as in the conventional solutions. However, in this case, the sheet of vitroceramic glass and the strip of laminated glass have to be joined by soldering, adhesive bonding or some other means, which is difficult in mass production, is aesthetically unsatisfactory and, above all, entails risks of breakage during operation, by reason of the differences in expansion of the two types of glass used.